In the case of solders of Sn—Pb system, it has been possible to carry out temperature-hierarchical bonding, which comprises soldering lead (Pb)-rich Pb-5Sn (melting point: 314-310° C.), Pb-10Sn (melting point: 302-275° C.), etc. as high temperature series solders at temperatures of approximately 330° C., and then bonding a Sn-37Pb eutectic mixture (melting point: 183° C.) as low temperature series solders without melting the first soldered portions. These solders have been capable of bonding Si chips, etc., which are flexible and high in deformability and thus is easy to break, to a substrate having a different coefficient of thermal expansion. Such a temperature-hierarchical bonding has been applied to semiconductor devices of chip die-bonding type, semiconductor devices of chip flip-chip-bonding type such as BGA, CSP, etc. That is, this means that a solder for use within the semiconductor device and a solder for bonding a semiconductor device itself to a substrate undergo temperature-hierarchical bonding.
In every field, there is now a growing tendency to make the solder lead-free.
Sn—Ag eutectic series (melting point: 221° C.), Sn—Ag—Cu eutectic series (melting point: 221°-217° C.) and Sn—Cu eutectic series (melting point: 227° C.) are now the mainstreams of Pb-free solders. A lower soldering temperature is desirable for the surface mounting in view of the heat resistance of parts, but actually it is approximately 235°-245° C. in the case of Sn—Ag—Cu eutectic series having a possibly lowest bonding temperature in view of the necessity for assuring a wettability to attain the reliability and also in view of considerable temperature fluctuations throughout a substrate in spite of using a furnace with distinguished temperature uniformity control. Thus, solders for the hierarchical bonding, which can withstand such a soldering temperature, must have a melting point of at least 250° C., but actually any Pb-free solders for use on the high temperature side of temperature-hierarchical bonding, which can be used in combination thereof, is not yet available. Sn-5Sb (melting point: 240°-232° C.) is a most possible Pb-free solder composition, but is not available for the temperature-hierarchical bonding because of melting thereof.
Au-20Sn (melting point: 280° C.) is known as a solder of higher temperature series, but it is so hard and expensive that its use is restricted to a narrow range. Particularly in bonding Si chips to a material having a different coefficient of thermal expansion and also in bonding of large chips, the Au-20Sn solder is not available, because it is so hard as to break Si chips with high possibility.